Methods and systems for using flame rectification to detect the presence of a burner flame

ABSTRACT

Systems and methods for detecting the presence of a burner flame using flame rectification are shown and described. A conductive flame sensor is positioned to conduct electricity to a burner flame when the burner is lit. The flame provides a conductive path to the burner conductive body and when operatively connected to an alternating current source, half-wave rectifies the current flowing to the sensor. A flame sensing circuit provides an output signal that is conditioned for use as an input to a flame indicator and/or a controller that is configured to shut off gas flow to the burner when no flame is present after an attempt at igniting the burner.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.63/252,663, filed on Oct. 6, 2021, the entirety of which is herebyincorporated by reference.

FIELD

This disclosure relates to systems for detecting the presence of aburner flame using flame rectification, and more specifically, improvedmethods and systems using flame rods and hot surface igniters formingpart of the rectified current pathway when a flame is present.

BACKGROUND

When operating combustion burners, it is desirable to provide some meansof determining whether a flame is present to ensure that uncombustedcombustion gas is not supplied to the burner and its surroundings anddoes not create an explosion hazard. One known device for detecting thepresence of a flame is a “flame rod” or “flame rectification rod.”

A flame rod is a conductive rod with a ceramic insulator which serves asa first electrode and is positioned to contact the flame when the burneris ignited. The burner housing serves a second electrode. When it ispresent, and an excitation voltage is supplied to the flame rod, theflame provides a conductive pathway that allows current to flow from theflame rod to the burner housing. Conversely, when an excitation voltageis supplied and no flame is present, no current flows from the flame rodto the burner housing. A sensing circuit is typically connected to theflame rod to detect the presence of current from the flame rod to theburner housing so that an indication that a flame is or is not presentmay be provided. The combustion process typically produces soot or otherdeposits that foul the flame rod. The deposits act as an insulator,increasing the impedance of the flame rod and reducing the current tothe burner at a given voltage. As a result, flame rods must be replacedor serviced at some frequency as their impedances reach too high alevel. Thus, a need has arisen for an improved means of using flamerectification to detect the presence of a burner flame.

SUMMARY

In accordance with a first aspect of the present disclosure, a burnerflame detection system is provided which comprises a conductive flamesensor comprising a conductive terminal and a flame sensing circuitcomprising a flame detection signal output node. The conductive flamesensor conductive terminal is positioned proximal to a burner having aconductive body. The burner has an ignited state and an unignited statesuch that when the burner is in the ignited state, the burner and theconductive flame sensor are in electrical communication with oneanother. The flame sensing circuit is configured to supply analternating current having a frequency of from about 24 kHz to about 300KHz to the conductive flame sensor conductive terminal, and when theburner is in an ignited state and the alternating current is supplied tothe conductive terminal, the flame sensing circuit generates a rectifiedcurrent from the conductive flame sensor conductive terminal to theburner.

In accordance with a second aspect of the present disclosure, a methodof determining if a burner is ignited is provided. The method uses aconductive flame sensor comprising a conductive terminal and positionedproximate a burner. The method comprises providing a flame sensingalternate current source operatively connected to the conductiveterminal, the alternating current having a frequency of from about 24kHz to about 300 kHz; and generating a rectified current from theconductive flame sensor to the burner when a source of the alternatingcurrent supplies the alternating current to the flame sensor conductiveterminal, and the burner is in an ignited state

In accordance with a third aspect of the present disclosure, a burnerflame detection system is provided which comprises a hot surface ignitercomprising a conductive pattern connected to a conductive terminal andpositioned proximal to a burner having a conductive body. The burner hasan ignited state and an unignited state, such that when the burner is inthe ignited state, the conductive terminal and the burner conductivebody are in electrical communication with one another. In a preferredexample, the burner flame detection system includes a flame sensingcircuit configured to supply a flame sensing alternating current to thehot surface igniter conductive terminal, wherein when the burner is inthe ignited state and the flame sensing alternating current is suppliedto the hot surface igniter conductive terminal, the flame sensingcircuit generates a rectified current from the hot surface igniterconductive terminal to the burner.

In accordance with a fourth aspect of the present disclosure, A methodof determining if a burner having a conductive body is ignited isprovided. The method comprises providing a conductive flame sensorhaving a conductive terminal and positioned proximate the burner andproviding a flame sensing alternate current source operatively connectedto the conductive terminal and having an alternating current with afrequency of from about 24 kHZ to about 300 kHz and generating arectified current from the conductive flame sensor to the burner whenthe flame sensing alternating current source supplies flame sensingalternate current to the conductive flame sensor's conductive terminal,and the burner is in an ignited state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depicting a burner flame detection system inaccordance with the present disclosure;

FIG. 2A is a schematic depicting a modification of FIG. 1 in which a hotsurface igniter's heating conductive pattern is also used as a flamesensing conductive pattern;

FIG. 2B is a schematic depicting a modification of FIG. 1 in which a hotsurface igniter includes both a heating conductive pattern and aseparate flame sensing conductive pattern.

FIG. 3A is a schematic depicting a direct current source used to supplya DC voltage to the oscillator circuit of FIG. 1 ;

FIG. 3B is a schematic depicting an alternating current source used tosupply a DC voltage to the oscillator circuit of FIG. 1 ;

FIGS. 4A-4D depict various examples of “third leg” hot surface igniterssuitable for use as flame sensors in the burner flame detection systemof FIG. 1 ;

FIGS. 5A-5C depict voltage versus time data for the emitter, collector,and base of the flame rod driver circuit bipolar junction transistor andfor the flame rod driver circuit capacitor output node of FIG. 1 whenthe flame sensing circuit is subjected to an alternating current and aburner flame is not present;

FIGS. 6A-6C depict voltage versus time data for the emitter, collector,and base of the flame rod driver circuit bipolar junction transistor andfor the flame rod driver circuit capacitor output node of FIG. 1 whenthe flame sensing circuit is subjected to an alternating current and aburner flame is present;

FIGS. 7A-7B are plots of simulated peak-to-peak and mean output voltage,respectively, versus frequency for a flame rod with a specified degreeof fouling;

FIGS. 7C-7D are plots of simulated mean and peak-to peak input voltage,respectively, versus frequency for the flame rod with the specifieddegree of fouling of FIGS. 7A-7B; and

FIGS. 8A-8C are plots of simulated peak-to-peak output voltage, meanoutput voltage, and mean input voltage, respectively, versus frequencyfor the fouled flame rod of FIGS. 7A-7B one week later

Like reference numerals refer to like parts in the figures.

DESCRIPTION

The systems and methods herein use the property of “flame rectification”to determine whether a burner flame is present. In flame rectification,an active flame defines an electrical path from a flame sensor to aburner body. As is known to skilled artisans, based on Mollberg's flamemodel, a conductive path through a flame may be modeled as a highresistance (megaohms) resistor in series with a diode. Thus, whensubjected to an alternating current, the flame conducts electricity whenthe AC signal is positive and acts as an open circuit when the AC signalis negative.

Flame rods are conductive rods that are typically used as flamerectification sensors. The rod is typically positioned inside the flamewhen the burner is lit. Over time, soot from the combustion process andother particulate matter accumulate on the flame rod and “foul it”causing it to diminish in its sensitivity. It has been found that aflame rod may be modeled as a capacitor. The accumulated deposits may bemodeled as an insulator of varying thickness. As is known to thoseskilled in the art, the complex impedance of an RC circuit is a vectorsum of a resistance and a “reactive capacitance”:

$\begin{matrix}{Z^{2} = {R^{2} + X_{c}^{2}}} & (1)\end{matrix}$ $\begin{matrix}{X_{c} = \frac{1}{2\pi{fC}}} & (2)\end{matrix}$

where Z=complex (vector) impedance (ohms)

R=resistance (ohms)

f=frequency (sec⁻¹)

C=capacitance (farads)

X_(c)=capacitive reactance

As equations (1) an (2) suggest, as deposits accumulate on a flame rod,its capacitance decreases, which increases the contribution of thereactive capacitance Xc to the impedance Z. At a standard (US) ACfrequency of 60 Hz, the contamination that develops on flame rodsproduces a significant impedance. As equations (1) and (2) also suggest,as the frequency f of an applied AC signal increases, the capacitivereactance Xc decreases, and the complex impedance approaches theresistance. It has been discovered that by sufficiently increasing thefrequency of an AC excitation signal supplied to a flame rod, thesensitivity of the flame rod's impedance to the accumulation of soot orother deposits can be significantly diminished.

Hot surface igniters are a well-known means of igniting combustion gas.Silicon nitride hot surface igniters typically comprise two insulatingtiles with a printed conductive, heat generating pattern printed on oneof the inside faces of the two insulating tiles. When connected to avoltage source, the conductive, heat generating pattern generates heat.It has also been discovered that a hot surface igniter of thisconstruction can also function as a flame rectifying sensor and bemodeled as a capacitor. Relative to flame rods, hot surface ignitershave the added advantage of generating combustion temperatures whichallows them to burn off accumulated deposits and avoid the replacementcycles that are necessary for flame rods. Disclosed herein are circuitsintended to generate a binary (ON/OFF) signal of a voltage rangesuitable for a commercial microcontroller based on the presence of aflame rectified signal generated when a flame rectification sensor isexposed to a flame.

Referring to FIG. 1 , a burner flame detection system 20 is depicted.Burner flame detection system 20 comprises conductive flame sensor 22,and a flame sensing circuit 24. Flame sensor 22 is preferably a flamerectification sensor positioned with respect to burner 21 such that whenburner 21 is lit, an electrical path exists from the flame sensor 22,through the flame to a conductive burner body comprising burner 21.

Flame sensing circuit 24 is designed to provide a signal at flamesensing circuit 24 flame detection signal output node 36 that issuitable for input to a flame presence indicator. The indicator ispreferably visual and/or audible. The flame presence indicator may be astand-alone indicator or may be integrated with a controller, such as acommercially available microcontroller. In preferred examples thecontroller is operatively connected to a gas valve that is operative toselectively supply combustion gas to burner 21. In certain preferredexamples, flame sensing circuit 24 is designed to provide a binarysignal (ON/OFF) at flame sensing circuit output node 36 even though thesignal generated by flame sensor 22 is not a binary signal. In theexample of FIG. 1 , flame sensing circuit 24 is designed to provide alogical high indication (e.g., 3.3V or 5 V) when no flame is present anda logical low (e.g., 0 V) when a flame is present.

Flame sensor 22 is preferably a flame rod or a hot surface igniter. Incertain examples in which flame sensor 22 is a hot surface igniter, theheating conductive pattern generates heat during an ignition operationand detects the presence of a flame during a flame detection operation.In other examples, the hot surface igniter includes a flame sensingconductive pattern separate from the heating conductive pattern so thatheating and flame sensing can occur simultaneously.

The flame sensing circuit 24 receives an AC voltage signal at input node38, which is an output from AC generating circuit 31. The AC signalgenerated by the AC generating circuit may be a sine wave but ispreferably a square wave.

In preferred examples, and as shown in FIG. 1 , the AC generatingcircuit 31 is a multi-vibrator oscillator circuit having a DC source 33and which generates an AC square wave at output node 34.

In the example of FIG. 1 , the multi-vibrator oscillator circuit is anastable oscillator circuit of the type known in the art. AC generatingcircuit 31 comprises four resistors 35 a-35 d, each having an input nodeconnected to DC source 33. Resistors 35 a and 35 d are each connected torespective output nodes 41 a and 41 b. Output node 41 a is connected tothe collector of bipolar junction transistor (BJT) 37 a and to capacitor39 b. Output node 41 b is connected to the collector of BJT 37 b and tocapacitor 39 a. Resistors 35 b and 35 c are connected to nodes 43 a and43 b, respectively. Resistor 35 b output node 43 a is also connected tothe base of BJT 37 a, which is also connected to capacitor 39 a.Resistor 35 c output node 43 c is also connected to the base of BJT 37b, which is also connected to capacitor 39 b.

When a DC source 33 supplies a DC voltage to resistors 35 a-35 d,capacitors 39 a and 39 b will charge and discharge. As capacitors 39 aand 39 b charge and discharge, BJTs 37 a and 37 b will alternate beingON and OFF, causing the BJT 37 a and 37 b collector voltages to rise andfall, thereby recharging capacitors 39 a and 39 b. Capacitor 39 c andresistor 35 e serve as a high-pass filter that removes the DC outputthat would otherwise be observed at AC generating circuit 31 output node34. DC source 33 provides a supply voltage of from about 10V to about48V, preferably from about 12V to about 36 V, and more preferably about24V. DC source 33 is shown in greater detail in FIG. 3A. The DC source33 comprises a battery 92 with a ground terminal 100 and a positiveterminal 93, which is connected to the AC generating circuit 31. In analternate implementation for situations where DC is not available, an ACconverting circuit of the type shown in FIG. 3B may be provided. The ACconverting circuit 49 comprises 24V AC supply 95 having a positiveterminal 96 and a ground terminal 100. Positive terminal 96 is connectedto diode 97 which is connected to output node 91 which is connected toAC generating circuit 31. Ripple capacitor 98 is provided between theoutput node 91 and ground. When the voltage of AC supply 95 is positiveand above the saturation voltage of capacitor 98, capacitor 98 charges.When the AC supply 95 voltage is below the saturation voltage ofcapacitor 98, capacitor 98 discharges, thereby smoothing the ripplecaused by diode 97 half wave rectifying the current from AC supply 95.

Capacitors 39 a and 39 b—along with resistors 35 b and 35 c—are selectedto achieve a desired frequency of the AC signal at output node 34 byadjusting the current flow across capacitors 39 a and 39 b. Resistors 35a and 35 d are selected to achieve a desired rising edge time of the ACsignal at output node 34 by adjusting the current through BJTs 37 a and37 b. In preferred examples, AC generating circuit 31 is a balancedmultivibrator with the resistances of resistors 35 b and 35 c beingequal, the resistances of resistors 35 a and 35 d being equal, and thecapacitances of capacitors 39 a and 39 b being equal.

In preferred examples, the component values of AC generating circuit 31are selected to produce a square wave AC voltage signal at output node34 having a frequency range of from 24 kHZ to 300 kHZ, more preferably40 kHz to 200 kHz, and still more preferably from 70 kHZ to 100 kHZ, andmore preferably from 80 kHz to 90 kHZ. In preferred examples, thesefrequencies yield a stable signal (logical high or low) at flame sensingcircuit output node 36 when burner 21 is lit. It has been found thatwhen flame sensor 22 is a flame rod and the AC signal at AC generatingcircuit output node 34 has a frequency in these ranges, the signal atflame sensing circuit output node 36 is stable when burner 21 is liteven when significant deposits have accumulated on the flame rod.Exemplary component values for AC generating circuit 31 for achieving hepreferred frequencies referenced above are as follows:

TABLE 1 More Even more Component Preferred preferred preferred Resistor35a 8 kΩ-12 kΩ 9 kΩ-11 kΩ 9.5 kΩ-10.5 kΩ Resistor 35b 80 kΩ-120 kΩ 90kΩ-110 kΩ 95 kΩ-105 kΩ Resistor 35c 80 kΩ-120 kΩ 90 kΩ-110 kΩ 95 kΩ-105kΩ Resistor 35d 8 kΩ-12 kΩ 9 kΩ-11 kΩ 9.5 kΩ-10.5 kΩ Capacitor 460pF-520 pF  470 pF-510 pF  480 pF-500 pF  39a/39b Resistor 35e 8 kΩ-12 kΩ9 kΩ-11 kΩ 9.5 kΩ-10.5 kΩ BJT 37a/BJT 0.5 V-0.9 V  0.55 V-0.85 V  0.6V-0.8 V  37b V_(BE) breakdown voltage BJT 37a/BJT 30 V-50 V  35 V-45 V 38 V-42 V  37b V_(CE) breakdown voltage

Flame sensing circuit 24 comprises a flame sensor driver circuit 26, asignal conditioning circuit 28, and a load circuit 30. The flame sensor22 includes a conductive terminal 32 that receives an output signal froman output node (not separately shown) of the flame sensor driver circuit26. In the case of a flame rod, conductive terminal 32 is electricallyconnected to the flame rod body. In the case of a hot surface igniter,conductive terminal 32 is electrically connected to a heating conductivepattern or a flame sensing conductive pattern in the hot surfaceigniter.

Flame sensor driver circuit 26 provides a means of amplifying a DCoffset introduced when the flame sensor 22 is subjected to analternating current while a flame is present. The flame sensor drivercircuit 26 comprises resistor 42 which is in series with flame sensor 22and which has an input node 45 connected to capacitor 40 and resistor44. Capacitor 40 has an input connected to input node 38. Input node 38is connected to BJT 46 emitter 48 and AC generating circuit output node34. BJT 46 emitter 48 is also the output node of the flame sensor driverdetection circuit 24. Resistor 44 is connected to BJT base 60 andresistor 42 input node 45. BJT 46 collector 50 defines a flame sensordriver circuit flame detection output node that is connected to signalconditioning circuit 28. BJT 46 acts as a switch that supplies currentfrom collector 50 to signal conditioning circuit 28 when a flame ispresent.

BJT 46 is a PNP BJT in which there is a positive offset from the emitter48 to base 60 having a fixed voltage when there is a path for currentflow through the base 60. When no flame is present, there is effectivelyan open circuit from flame sensor 22 to burner 21, and there is no pathfor current flow from emitter 48 to base 60. However, when a flame ispresent, and there is a path for current flow through the base toresistor 42, BJT 46 is turned ON, which allows current to flow fromemitter 48 to collector 50. When there is no flame, collector 50 floatson the emitter 48 voltage during the positive AC cycle and is connectedto ground during the negative AC cycle. Current flow through resistor 42and flame sensor 22 produces a DC offset voltage at node 45 which causesthe BJT 46 offset between emitter 48 and base 60 to exceed the thresholdrequired to turn BJT 46 ON. Preferred examples of component values forflame sensor driver circuit 26 are as follows:

TABLE 2 More Even more Component Preferred preferred preferred Capacitor40 0.003 μF-0.014 μF  0.004 μF-0.013 μF  0.008 μF-0.011 μF  Resistor 4280 kΩ-120 kΩ 90 kΩ-110 kΩ 95 kΩ-105 kΩ Resistor 44 80 kΩ-120 kΩ 90kΩ-110 kΩ 95 kΩ-105 kΩ BJT 46 −440 V to −390 V −430 V to −370 V −420 Vto −380 V V_(CE) breakdown voltage

When a flame is present, with BJT 46 ON current flows from the ACgenerating circuit output node 34 to diode 52, which is part of signalconditioning circuit 28. Signal conditioning circuit 28 includes an RClow pass filter and converts the input signal at diode 52 to a positiveand less variable signal at signal conditioning circuit output node 66when a flame is present. The reduced variability ensures that the loadcircuit 30 can more reliably supply a DC voltage having discrete binaryvalues at load circuit output node 36, which is also the flame sensingcircuit 24 output node.

Signal conditioning circuit 28 comprises diode 52 which his connected toa parallel combination of resistor 58 and capacitor 54. Resistor 58 andcapacitor 54 define an RC low pass filter. When the AC signal at BJT 46collector 50 is positive, diode 52 is forward-biased and allows currentto pass. When the AC voltage at BJT 46 collector 50 is negative, diode52 is reverse-biased and does not allow current to pass. Thus, resistor58 and capacitor 54 only see positive voltages. When the AC voltage atBJT 46 collector 50 is more than the voltage of capacitor 54, capacitor54 charges until reaching its peak voltage. When the AC voltage at BJT46 collector 50 is below the capacitor voltage, capacitor 54 discharges.Thus, the capacitor 54 smooths the ripple created by the half-waverectification provided by diode 52. Resistor 58 provides a path toground to remove excess charge across capacitor 54. Resistor 58 inputnode 56 is connected to current limiting resistor 62.

Signal conditioning circuit 28 also comprises a current limitingresistor 62 which, along with Zener diode 64, is connected to signalconditioning circuit output node 66. Zener diode 64 is reverse biasedand protects load circuit 30 against power surges because once itreaches its breakdown voltage, Zener diode 64 allows current to flow toground, thus capping the signal conditioning circuit output node 66 atthe breakdown voltage. It is generally desirable to maximize the voltageat the signal conditioning circuit output node 66. Thus, diode 52 ispreferably selected to have a small voltage drop. It is also desirableto filter out the AC noise and have a faster response at signalconditioning circuit output node 66, therefore, capacitor 54 is selectedto have a low capacitance. Resistor 62 is preferably selected to have aresistance that will protect against voltage surges at signalconditioning circuit output node 66, and Zener diode 64 is selected tohave a breakdown voltage at the maximum desired voltage at signalconditioning circuit output node 66.

Preferred examples of component values for signal conditioning circuit28 are shown in Table 3:

TABLE 3 More Even more Component Preferred preferred preferred Diode 52forward 0.6 V-1.5 V 0.8 V-1.4 V 0.9 V-1.3 V voltage drop Capacitor 540.003 μF-0.014 μF 0.004 μF-0.013 μF 0.008 μF-0.011 μF Resistor 58  8MΩ-12 MΩ  8.5 MΩ-11.5 MΩ  9 MΩ-11 MΩ Resistor 62 0.7 MΩ-1.3 MΩ 0.8MΩ-1.2 MΩ 0.9 MΩ-1.1 MΩ Zener diode 64 10 V-14 V 11 V-13 V 11.5 V-12.5 Vbreakdown voltage

Signal conditioning circuit 28 is connected to load circuit 30. In theexample of FIG. 1 , the signal conditioning circuit 28 is connected tothe gate of MOSFET 68. MOSFET 68 is preferably an n-type MOSFET in whichthe source 70 is connected to ground and the drain 72 is connected tonode 77. Because it is an n-type MOSFET, the channel current flows fromthe drain to the source when the MOSFET is ON.

DC source 74 supplies direct current to load circuit 30. Load circuit 30adjusts the voltage from the signal conditioning circuit 28 to match theinput requirements of a controller connected to flame sensing circuitoutput node 36. In an alternate implementation, DC source 33 suppliesboth the AC generating circuit 31 and the load circuit 30. In certainexamples, the voltage at signal conditioning circuit output node 66ranges from 0-25V, preferably from 0-12V, and more preferably from 0-9V,depending on the frequency of the voltage signal at AC generatingcircuit output node 34.

The output from node 66 of the signal conditioning circuit 28 is not acurrent that flows into the load circuit 30, but rather a voltage thatturns the MOSFET 68 ON and OFF. When MOSFET 68 is ON it acts like a lowresistance resistor, providing a low impedance path to ground. As aresult, without Zener diode 78 and resistor 80, the input voltage to acontroller (i.e., the voltage at flame sensing circuit output node 36)would be approximately zero when a flame is present, and approximatelythe voltage of DC supply 74 (e.g., 24V) when no flame is present, whichis too high for many commercially available microcontrollers. Zenerdiode 78 and resistor 80 form a parallel combination and are selected tomatch the microcontroller (not shown) input requirements. Zener diode 78also protects the controller against excessive voltages by effectivelycapping the voltage at flame sensing circuit output node 36 at thebreakdown voltage of Zener diode 78.

Using the following relationship, the resistance of resistor 80 can bedetermined based on a desired maximum input voltage to the controller:

$\begin{matrix}R_{{80} = \frac{R_{76}}{({\frac{V_{74}}{V_{80}} - 1})}} & (3)\end{matrix}$

-   -   where, R₈₀=resistance of resistor 80 (ohms)    -   V₇₄=DC input voltage from DC source 74 (volts)    -   V₈₀=maximum controller input voltage (volts)

In an example where R₇₆ is 47 kΩ, and the maximum controller inputvoltage is 3.3V, equation (3) yields a resistance of 7.5 kΩ for R₈₀.Zener diode 78 may also be selected to have a breakdown voltage equal tothe maximum input voltage of the microcontroller to protect it fromsurges. Preferred exemplary component values for the load circuit 30 areprovided in Table 4:

TABLE 4 More Even more Component Preferred preferred preferred MOSFET 68(drain 2.5 Ω-5.5 Ω 3 Ω-5 Ω 3.5 Ω-4.5 Ω to source resistance when MOSFETis ON) MOSFET 68 (gate 0.8 V-3.3 V 0.8 V-2.5 V 0.8 V-1.8 V thresholdvoltage) Resistor 76 35 kΩ-60 kΩ 40 kΩ-55 kΩ 45 kΩ-50 kΩ Resistor 80  6kΩ-10 kΩ 6.5 kΩ-9.5 kΩ 7 kΩ-9 kΩ Zener diode 64 2.0 V-5.5 V 2.5 V-4 V  3.0 V-3.5 V breakdown voltage

As mentioned previously, in certain examples, a hot surface igniter maybe used in place of a flame rod for flame sensor 22. The hot surfaceigniter comprises at least two ceramic insulating tiles having a heatingconductive pattern disposed between them, such as by printing thepattern on one of the inner faces of one of the insulating tiles. Inpreferred examples, the ceramic tiles comprise silicon nitride. Examplesof such silicon nitride igniters are shown in U.S. patent applicationSer. No. 16/366,479, the entirety of which is hereby incorporated byreference.

In certain examples, silicon nitride igniters comprising only a heatingconductive pattern are used as flame sensor 22, while in other exampleshot surface igniters are provided which include both a heatingconductive pattern and a separate flame sensing conductive pattern thatis used as flame sensor 22. In the former case, the igniter operates inboth a heating mode and a flame sensing mode. In the heating mode, theheating conductive pattern generates heat when a voltage is appliedacross its conductive terminals. In the flame sensing mode, a voltage isnot applied across the terminals of the igniter. Instead, one of theterminals is connected to resistor 42 of flame sensor driver circuit 26and the other is disconnected from ground so that the flame sensingconductive circuit acts as an electrode that conducts electricity to theburner flame when the burner 21 is lit. A schematic illustrating animplementation of this type of igniter is shown in FIG. 2A.

In FIG. 2A igniter 84 is represented as a resistor. The heatingconductive pattern (not separately shown) is connected to a positiveterminal 113 a and a ground terminal 113 b. Switches 86 and 88 areselectively openable to disconnect igniter 84 from an AC mains supply 82and selectively closable to connect igniter 84 to mains AC supply 82, inwhich case AC mains supply acts as a source of ignition alternatingcurrent. AC mains supply 82 has a positive terminal 83 and a groundterminal 85. Switches 86 and 88 preferably define a single throw, doublepole switch assembly in which one actuator simultaneously opens switches86 and 88 or simultaneously closes switches 86 and 88.

In a heating mode, switches 86 and 88 are in a closed position and makeelectrical contact with positive igniter terminal 113 a and groundigniter terminal 113 b, respectively. In the heating mode, current flowsfrom AC mains supply 82 positive terminal 83 through igniter 84 and tonode 90 which connects igniter ground terminal 113 b to AC mains groundterminal 85. In a flame sensing mode, switches 86 and 88 are in an openposition so that the igniter 84 is disconnected from both AC mainssupply 82 and ground. It has been discovered that disconnecting groundterminal 113 b from ground is important when igniter 84 is in a flamesensing mode because otherwise current from the flame sensor drivercircuit 26 may short to ground instead of flowing through the burnerflame and to the body of burner 21.

Referring to FIG. 2B, an exemplary implementation of “third leg” hotsurface igniters is provided. The phrase “third leg hot surfaceigniters” refers to igniters containing both a heating conductivepattern and a flame sensing conductive pattern. As illustrated in FIGS.4A-4D, the heating conductive pattern (e.g., heating conductive patterns116, 136, 156, 170 in FIGS. 4A-4D), can be sandwiched between insulatingtiles of hot surface igniter 84. The flame sensing conductive patternmay be located between two outermost insulating tiles or on the outsideof one of the outermost insulating tiles. The flame sensing conductivepattern is electrically isolated from the heating conductive pattern andincludes a single conductive terminal 32 that is connected to the outputof the flame sensing driver circuit resistor 42 (i.e., flame sensingdriver circuit flame sensor output node) and the flame sensingconductive pattern. The heating conductive pattern is connected topositive terminal 92 and ground terminal 94. The positive terminal 92 isselectively connectable to AC mains supply 82 by selectively opening andclosing switch 86, which is connected to positive terminal 83 of ACmains supply 82. Ground terminal 94 is connected to ground as is groundterminal 89 of AC mains supply 82, and igniter ground terminal 94connects to AC mains ground terminal 89 at node 90. During a heatingoperation, switch 86 is closed to place positive terminal 92 of hotsurface igniter 84 in electrical communication with positive terminal 83of AC mains supply 82. During a flame sensing operation, if a heatingoperation is not concurrently in progress, switch 86 will remain open.The heating conductive pattern and flame sensing conductive pattern areelectrically isolated from one another such that the igniter groundterminal 94 can remain connected to ground without shorting out theelectrical the through the burner 21 flame when burner 21 is lit.

Referring to FIGS. 4A-4D several examples of third leg hot surfaceigniters 102, 120, 140, and 160 are shown. Each of igniters 102, 120,140, and 160 includes at least two ceramic, insulating tiles with aheating conductive pattern disposed between the at least two tiles. Eachof igniters 102, 120, 140, and 160 also includes a flame sensingconductive pattern disposed between the at least two ceramic insulatingtiles or on an outer face of one of the at least two ceramic insulatingtiles. The ceramic tiles are preferably formed from the compositions andhave the dimensions of those described in U.S. patent application Ser.No. 16/366,479 (the '479 App). The heating conductive patterns describedherein may be formed from the same materials and using the sameprocesses described in the '479 App. The flame sensing conductivepatterns may also be formed from the same materials and using the sameprocesses as described in the '479 App., but need not have the samepatterns as they act only as an electrode for conducting electricity toa flame when burner 21 is lit. They also need not be formulated togenerate heat unless the same conductive pattern is used for heatgeneration and flame sensing.

As discussed in the '479 App., ceramic hot surface igniters used in thegas burner systems described herein are prepared by sintering ceramiccompositions. In certain examples, post-sintering, the ceramicinsulating tiles used to form the igniter (not including conductive inkcircuit) have a room temperature resistivity that is no less than 10¹²Ω-cm, preferably no less than 10¹³ Ω-cm, and more preferably, no lessthan 10¹⁴ Ω-cm. In the same or other examples, the tiles have a thermalshock value in accordance with ASTM C-1525 of no less than 900° F.,preferably no less than 950° F., and more preferably, no less than 1000°F.

In other examples, the conductive ink comprising the heating conductivepattern has a (post-sintering) room temperature resistivity of fromabout 1.4×10⁻⁴ Ω·cm to about 4.5×10⁻⁴ Ω·cm, preferably from about1.8×10⁻⁴ Ω·cm to about 4.1×10⁻⁴ Ω·cm, and more preferably from about2.2×10⁻⁴ Ω·cm to about 3.7×10⁻⁴ Ω·cm. In the case of a material with aconstant cross-sectional area along its length, resistivity ρ at a giventemperature T is related to resistance R at the same temperature T inaccordance with the well-known formula:

R(T)=ρ(T)(l/A), where  (4)

-   -   ρ=resistivity of conductive circuit material (Ω-cm) at        temperature T;    -   R=Resistance in ohms (Ω) at temperature T;    -   T=Temperature (° F. or ° C.);    -   A=cross-sectional area (cm²) of conductive ink circuit        perpendicular to the direction of current flow; and    -   l=total length (cm) of the conductive ink circuit along the        direction of current flow.

In the case of a cross-sectional area that varies along the length ofthe conductive circuit, the resistance may be represented as:

$\begin{matrix}{R = {{\rho(T)}{\int_{0}^{L}\frac{dl}{A}}}} & (5)\end{matrix}$

where, L=total length of circuit along direction of current flow (cm),and the remaining variables are as defined for equation (4).

In certain examples, the ceramic bodies comprising the ceramic hotsurface igniters described herein preferably comprise silicon nitrideand a rare earth oxide sintering aid, wherein the rare earth element isone or more of ytterbium, yttrium, scandium, and lanthanum. Thesintering aids may be provided as co-dopants selected from the foregoingrare earth oxides and one or more of silica, alumina, and magnesia. Asintering aid protective agent is also preferably included which alsoenhances densification. A preferred sintering aid protective agent ismolybdenum disilicide. The rare earth oxide sintering aid (with orwithout the co-dopant) is preferably present in an amount ranging fromabout 2 to about 15 percent by weight, more preferably from about 8 toabout 14 percent by weight, and still more preferably from about 12 toabout 14 percent by weight of the ceramic body. Molybdenum disilicide ispreferably present in an amount ranging from about 3 to about 7 percent,more preferably from about 4 to about 7 percent, and still morepreferably from about 5.5 to about 6.5 percent by weight of the ceramicbody. The balance is silicon nitride.

The conductive ink circuit is preferably printed onto the face of one ofthe ceramic tiles to yield a ceramic hot surface igniter(post-sintering) with heating properties that are tailored to thespecific application for which the igniter is intended as well as to thevoltage at which the igniter will operate. Listed below in Table 5 aresome exemplary room temperature resistance (RTR) values for variousapplications.

TABLE 5 Supply Voltage Preferred RTR Application (Volts) RTR Range(Ohms) Range (Ohms) HVAC 120 40-52 44-48 Oven 120 32-42 36-39 Hot WaterHeater 120 32-42 36-39 Hot Water Heater 230 130-170 145-155

The conductive ink used for the heating conductive circuit may comprisetungsten carbide in an amount ranging from about 20 to about 80 percent,preferably from about 30 percent to about 80 percent, and morepreferably from about 70 to about 75 percent by weight of the ink.Silicon nitride is preferably provided in an amount ranging from about15 to about 40 percent, preferably from about 15 to about 30 percent,and more preferably from about 18 to about 25 percent by weight of theink. The same sintering aids or co-dopants described for the ceramicbody are also preferably included in an amount ranging from about 0.02to about 6 percent, preferably from about 1 to about 5 percent, and morepreferably from about 2 to about 4 percent by weight of the ink. Incertain examples, the flame sensing conductive pattern comprises isformed from an ink of the same composition as the heating conductivepattern.

Referring to FIG. 4A, an exploded view of hot surface igniter 102 isprovided. Igniter 102 comprises first ceramic insulating tile 104 andsecond ceramic insulating tile 106. First ceramic insulating tile 104has an inner face 110 a and an outer face 110 b. Second ceramicinsulating tile 106 has an outer face 108 a and an inner face 108 b. Theinner faces 110 a and 108 b of the first ceramic insulating tile 104 andsecond ceramic insulating tile 106 face one another. Heating conductivepattern 116 is printed on inner face 108 b of second insulating ceramictile 106 and is connected to conductive terminal 32. Flame sensingconductive pattern 112 is printed on outer surface 100 b of firstinsulating ceramic tile 104. The insulating ceramic tiles 104 and 106are laminated as described in the '479 Application to create a unitaryhot surface igniter structure.

Referring to FIG. 4B, hot surface igniter 120 comprises first ceramicinsulating tile 122, second ceramic insulating tile 124, and thirdceramic insulating tile 126. First ceramic insulating tile 122 includesan inner face 128 b and an outer face 128 a, and the flame sensingconductive pattern is printed on the inner face 128 b. Conductiveterminal 32 is connected to flame sensing conductive pattern 138.

Second insulating ceramic tile 124 includes a first face 130 a thatfaces ceramic insulating tile 122, and a second face 130 b that facesthird insulating ceramic tile 126. Heating conductive pattern 136 isprinted on the second face 130 b of the second ceramic insulating tile126 and faces the inner face 132 a of third ceramic insulating tile 132a. The heating conductive pattern is connected to two terminals 134 aand 134 b for connection to a power source and ground, respectively. Thethree ceramic insulating tiles 122, 124, and 126 are laminated togetherusing the techniques described in the '479 App. to create a unitary hotsurface igniter structure.

In the example of FIG. 4C, hot surface igniter 140 includes threeceramic insulating tiles 142, 144, and 146. A flame sensing conductivepattern (not shown, but similar to the patterns 112 and 138 of FIGS. 4Aand 4B) is connected to conductive terminal 32 and is printed on a firstface 150 a of ceramic insulating tile 144. Heating conductive pattern156 is printed on a second face 150 b of ceramic insulating tile 144.Both the flame sensing pattern and the heating conductive pattern 156are sandwiched between ceramic insulating tiles 142 and 144. Ceramicinsulating tile 142 has an outer face 148 a and an inner face 148 b.Inner face 148 b faces the face 150 a of ceramic insulating tile 144.Ceramic insulating tile 146 has an inner face 152 a and an outer face152 b. Inner face 152 a faces face 150 b of ceramic insulating tile 144.The three ceramic tiles 142, 144, and 146 are laminated using thetechniques in the '479 App. to create a unitary hot surface igniter.

In the example of FIG. 4D, hot surface igniter 140 includes a firstceramic insulating tile 162 and a second ceramic insulating tile 165.First ceramic insulating tile 162 has an outer face 164 a and an innerface 164 b. Second ceramic insulating tile 164 has an inner face 166 aand an outer face 16 b. However, in this example, both the heatingconductive pattern 170 and the flame sensing conductive pattern 172 areprinted on the same face 164 b of first ceramic insulating tile 160 andface the inner face 166 a of second ceramic insulating tile 164. Theceramic insulating tiles 162 and 164 are laminate using the techniquesdescribed in the '479 App. to create a unitary hot surface igniter.

Example 1 Simulation of Flame Sensor Driver Circuit

In this example, the operation of the flame sensor driver circuit 26 ofFIG. 1 is illustrated both when burner 21 is lit and when it is not lit.The component values used for the simulation are as follows:

Component Value Flame Sensor Driver Circuit BJT 46 V_(CE) breakdown −400v voltage Capacitor 40 Capacitance 40 μF Resistor 42 100 kΩ Resistor 44100 kΩ Signal Conditioning Circuit Diode 52 forward voltage 1.1 V dropCapacitor 54 0.01 μF Resistor 58 10 M Ω Resistor 62 1 M Ω Zener diode 64breakdown 12 V voltage

A 24 kHZ voltage signal is supplied to the emitter 48 of BJT 46, andvoltages at node 45, BJT collector 50 and BJT base 60 are determined viasimulation. FIG. 5A shows the input voltage signal 180 at emitter 48 andthe voltage signal 182 at node 45. FIG. 5B shows the input voltagesignal 180 and the voltage signal 184 at BJT base 60. FIG. 5C shows theinput voltage signal 180 at emitter 48 and the voltage signal 190 at BJTcollector 50.

When burner 21 is not lit, there is an open circuit between flame sensor22 and burner 21. Because there is no current path through resistor 42,there is no current path available from BJT emitter 48 to BJT base 60.Thus, there is no emitter-base offset in BJT 46, and BJT remains OFF. Asa result, the node 45 voltage floats on the emitter 48 voltage, andsignals 180 and 182 are the same. During positive AC cycles at emitter48, collector 50 is remains 0V and is essentially grounded. Duringnegative AC cycles, collector 50 sees the voltage of emitter 48. Thus,as shown in FIG. 5C the collector 50 voltage signal 190 closely tracksthe input voltage signal 180 during negative AC cycles, but duringpositive AC cycles, the collector 50 voltage is capped at zero. Becausethe voltage signal at collector 50 is never positive when burner 21 isnot lit—and because of diode 52—no current flows to the signalconditioning circuit 28, and the signal conditioning circuit 28 outputnode 66 will see ground.

When burner 21 is lit, there is an active current path from flame sensor22 to burner 21 which, in accordance with Mollberg's flame model, can bemodeled as a relatively lower resistance resistor in series with adiode, the series combination of which is in parallel with a relativehigher resistance resistor. As a result, positive current passes fromthe flame sensor 22 to the burner 21, but only a small negative leakagecurrent passes from the burner 21 to the flame sensor 22.

When the positive current flows from the flame sensor 22 to the burner21, the rectification effect of the flame causes a negative(time-varying) DC offset in the voltage at node 45 relative to node 38because capacitor 40 (like capacitors in general) cannot pass DC. FIG.6A shows the input voltage signal at node 38 and the voltage at node 45.The gap between the input signal 180 and the node 45 signal 194 at thecycle peaks represents this negative DC offset. A slightly lower offsetis present between BJT emitter 48 and BJT base 60 than between BJTemitter 48 and node 45 because of the voltage drop across resistor 44 asshown by the gaps between the input voltage signal 180 and the BJTcollector signal 198 in FIG. 6B. When BJT 48 is ON, the path from theemitter 48 to collector 50 is a very low impedance path. Thus, in FIG.6C the BJT collector signal 202 closely tracks the input voltage signal180 at node 38 (and emitter 48). However, only positive voltages willpass signal conditioning circuit diode 52.

As illustrated by FIGS. 5A-5C and 6A-6C, when burner 21 is lit, thesignal conditioning circuit 28 sees the positive voltage cycles of ACgenerating circuit 31, but not the negative ones, and the values of thepositive AC cycle voltages are smoothed and made more constant at signalconditioning circuit output node 66 because of the low-pass filtering ofthe RC network in signal conditioning circuit 28. As a result, thetime-varying DC offset generated by the flame when burner 21 is lit isconverted to something much closer to a binary ON/OFF signal, albeit onewith values that may not be suitable for certain commercially availablemicrocontrollers. Thus, load circuit 30 is provided to convert thatsignal to a DC voltage with a logical high and low that match themicrocontroller input requirements.

Example 2 Determination of AC Frequency for Reducing the Effects ofFlame Rod Fouling

As discussed previously, higher frequency AC voltages supplied top flamerod can reduce the impact of accumulated deposits on the impedance ofthe flame rod. They can also reduce the impact of the ceramic insulatingtiles of a hot surface igniter on the igniter's impedance. In thisexample, the circuit of FIG. 1 is used with a flame rod serving as flamesensor 22, except that AC generating circuit 31 is replaced with anarbitrary wave form generator (AWG), and the load circuit is modified toeliminate resistor 80 and Zener diode 78 and to add an LED betweenresistor 76 and MOSFET 68. As the square wave frequency from the AWG isvaried, the percentage of the time the LED is ON is assessed when aflame is present. The goal is to have the LED be on 100 percent of thetime that burner 21 is lit. The component values are as follows:

Component Value Flame Sensor Driver Circuit BJT 46 V_(CE) breakdown −400v voltage Capacitor 40 Capacitance 40 μF Resistor 42 100 kΩ Resistor 44100 kΩ Signal Conditioning Circuit Diode 52 forward voltage 1.1 V dropCapacitor 54 0.01 μF Resistor 58 10 M Ω Resistor 62 1 M Ω Zener diode 64breakdown 12 V voltage

A “Monster Mash” is a formula used to simulate extreme soilingconditions by mixing a wide variety of diverse food ingredientstypically used in—and creating soils in—household ovens. In thisexample, the Monster Mash comprises cherry pie filling, tomato puree,egg yolks, whole milk mozzarella cheese, pasteurized cheese spread,lard, and tapioca. The Monster Mash is applied along 2.249 inches (57.13mm) of a flame rod having a length of 3.249 inches (82.53 mm) and adiameter of 0.114 inches (2.89 mm). It is applied by running the flamerod through a volume of the Monster Mash applied to a flat surface toobtain a thin layer that is slightly translucent and smoothed around theflame rod until even. The flame rod is placed in a pan but supportedabove the surface of the pan to prevent burning and placed in an ovenpreheated to 375° F. for 7-8 minutes, until the Monster Mash is goldenbrown. The baked layer diameter of the flame rod is 0.127 inches (3.23mm). Burner 21 is lit, the AWG frequency is varied from 10 Hz to 3 MHz,and the “on-rate” is measured. The peak to peak voltage at flame sensordriver circuit input node 38 is measured as is the mean voltage. Thepeak to peak voltage at signal conditioning circuit output node 66 isalso measured as is the mean voltage.

FIGS. 7A-D show the results of a first set of runs. FIGS. 7A and B showthat the peak-to-peak voltage and mean voltage at output node 66 do notrespond to the burner being lit until the frequency of the input voltageat flame sensor driver circuit input node 38 (which is also the flamesensing circuit input node) reaches a transition at about 10 kHz. Atlower frequencies, due to the fouling of the Monster Mash, the flamesensor driver circuit 26 mean voltage at output node 66 is expected tobe about zero, as is observed below 10 KHz (FIG. 7B). At 10 KHz thesignal conditioning circuit 28 output node 66 voltage switches rapidlybetween zero and 8V. At 700 kHZ, the output node 66 mean voltage variesbetween 3.5V and 8V. Above 700 kHZ the output node 66 voltage signalbecomes less stable, and the output node 66 mean voltage varies by up to2V (not shown in FIG. 7B). The LED is observed to be ON 100 percent ofthe time at frequencies between 200 kHZ and 400 kHZ. The minimumfrequency that yields a consistent output is 24 kHz, which yields a 95percent on rate.

FIGS. 8A-C show the results of a second set of runs performed a weekafter the first set of runs. FIGS. 8A and B show that the peak-to-peakvoltage and mean voltage at output node 66 do not respond to the burnerbeing lit until the frequency of the input voltage at flame sensordriver circuit input node 38 reaches a transition at about 2 kHz. In the2 kHz transition, the signal conditioning circuit output node 66 voltageswitches rapidly between 0 and 8V. The lack of a reliable peak-to-peakvoltage signal at output node 66 below 2 kHz (FIG. 8A) is indicative ofthe presence of fouling.

The LED is ON 99 percent of the time once the frequency reaches 20 kHZbut is not consistently lit 100 percent of the time except when thefrequency is in the range of 40 kHz-2 MHz. Because it is not rectified,the voltage at node 38 remains at zero or very close thereto during theentirety of the runs (FIG. 8C).

The differences in the runs of FIGS. 7A-7D compared to those in FIGS.8A-8C are believed to be attributable to inconsistencies in applying theMonster Mash and due to the length of the tests. However, based on thedata it is believed that a frequency range of 24 kHZ to 300 kHZ ispreferred, 40 kHZ to 200 kHz is more preferred and that a frequencyrange of 70 kHZ to 100 kHZ is still more preferred.

What is claimed is:
 1. A burner flame detection system comprising: aconductive flame sensor comprising a conductive terminal and positionedproximal to a burner having a conductive body, the burner having anignited state, and an unignited state, such that when the burner is inthe ignited state, the burner and the conductive flame sensor are inelectrical communication with one another; and a flame sensing circuitcomprising a flame detection signal output node, wherein the flamesensing circuit is configured to supply an alternating current having afrequency of from about 24 kHz to about 300 KHz to the conductive flamesensor conductive terminal, and when the burner is in the ignited stateand the alternating current is supplied to the conductive terminal, theflame sensing circuit generates a rectified current from the conductiveflame sensor conductive terminal to the burner.
 2. The burner flamedetection system of claim 1, further comprising a switch that isoperable to operatively connect a source of the alternating current tothe flame detection signal output node when the burner is in the ignitedstate.
 3. The burner flame detection system of claim 1, wherein theflame sensing circuit comprises an input node, a conductive flame sensordriver circuit, and a signal conditioning circuit having a signalconditioning circuit output node.
 4. The burner flame detection systemof claim 3, wherein the conductive flame sensor driver circuit comprisesa flame sensor driver circuit input node, a flame sensor driver circuitflame sensor output node, a flame sensor driver circuit flame detectionoutput node, a capacitor, and a bipolar junction transistor (BJT) havinga collector an emitter and a base, and the flame sensor driver circuitinput node is connected to the BJT emitter and the capacitor.
 5. Theburner flame detection system of claim 1, wherein the alternatingcurrent has a square wave waveform.
 6. The burner flame detection systemof claim 1, wherein the conductive flame sensor is a hot surfaceigniter.
 7. The burner flame detection system of claim 6, wherein thehot surface igniter comprises first and second insulating tiles and aheating conductive pattern located between the first and secondinsulating tiles, and the heating conductive pattern is connected to theconductive terminal.
 8. The burner flame detection system of claim 7,wherein the hot surface igniter comprises a flame sensing conductivepattern.
 9. The burner flame detection system of claim 1, wherein theconductive flame sensor is a flame rod.
 10. A burner flame detectionsystem, comprising: a hot surface igniter comprising a conductivepattern connected to a conductive terminal and positioned proximal to aburner having a conductive body, the burner having an ignited state andan unignited state, such that when the burner is in the ignited state,the conductive terminal and the burner conductive body are in electricalcommunication with one another.
 11. The burner flame detection system ofclaim 10, further comprising a flame sensing circuit configured tosupply a flame sensing alternating current to the hot surface igniterconductive terminal, wherein when the burner is in the ignited state andthe flame sensing alternating current is supplied to the hot surfaceigniter conductive terminal, the flame sensing circuit generates arectified current from the hot surface igniter conductive terminal tothe burner.
 12. The burner flame detection system of claim 10, furthercomprising a flame detection signal output node and a switch that isoperable to operatively connect a flame sensing alternate current sourceto the flame detection signal output node when the burner is in theignited state.
 13. The burner flame detection system of claim 11,wherein the flame sensing circuit comprises a flame sensor drivercircuit having an input node, and a signal conditioning circuit having asignal conditioning circuit output node.
 14. The burner flame detectionsystem of claim 13, further comprising a flame sensing alternatingcurrent source having a frequency of from about 24 kHz to about 300 KHzconnected to the flame sensor driver circuit input node.
 15. The burnerflame detection system of claim 14, wherein the flame sensingalternating current has a square wave waveform.
 16. A method ofdetermining if a burner is ignited using a conductive flame sensorcomprising a conductive terminal and positioned proximate the burner,the method comprising: providing a flame sensing alternate currentsource operatively connected to the conductive terminal, the alternatingcurrent having a frequency of from about 24 kHz to about 300 kHz; andgenerating a rectified current from the conductive flame sensor to theburner when the flame sensing alternating current source supplies thealternating current to the conductive flame sensor conductive terminal,and the burner is in an ignited state.
 17. The method of claim 16,wherein the conductive flame sensor is a flame rod.
 18. The method ofclaim 16, wherein the conductive flame sensor is a hot surface igniterhaving a conductive pattern connected to the conductive terminal. 19.The method of claim 18, further comprising the step of selectivelyconnecting the conductive terminal to a source of ignition alternatingcurrent.
 20. The method of claim 18, wherein the conductive pattern is afirst conductive pattern, the hot surface igniter comprises a secondconductive pattern, and the second conductive pattern is electricallyisolated from the first conductive pattern and connected to a source ofignition alternating current.
 21. A method of determining if a burnerhaving a conductive body is ignited, the method comprising: providing ahot surface igniter positioned proximate the burner and having aconductive pattern connected to a conductive terminal; providing a flamesensing alternate current source operatively connected to the conductiveterminal; and generating a rectified current from the hot surfaceigniter conductive pattern to the burner when the flame sensingalternating current source supplies flame sensing alternating current tothe hot surface igniter conductive terminal, and the burner is ignited.22. The method of claim 21, wherein the conductive terminal is a firstconductive terminal, and the conductive pattern is connected to a secondconductive terminal, the method further comprising the step ofselectively connecting the first conductive terminal to a source ofignition alternating current and the second conductive terminal toground.
 23. The method of claim 22, further comprising selectivelydisconnecting the first conductive terminal from the source of ignitionalternating current and selectively disconnecting the second conductiveterminal from ground.
 24. The method of claim 21, wherein the conductivepattern is a first conductive pattern, the hot surface igniter comprisesa second conductive pattern, and the second conductive pattern iselectrically isolated from the first conductive pattern and connected toa source of ignition alternating current.